Cam timing and interference engines

[C M Smith <cmsmith@domain.elided>]

Valve timing should not affect clearance between valveheads and pistons at 
TDC. Valve lift definitely makes a difference. The two should not be confused.

Valve timing is far more influential in terms of performance gains on 
street engines. Valve lift is considerably more difficult to change 
reliably without changing such things as the valve springs. Unfortunately, 
regrinding the existing cam often increases valve lift as the usual 
technique is to regrind the heel of the cam, and perhaps reprofile the ramp 
of the cam a tad. It is far better to get a proper custom cam grind from 
a  new billet preserving the original maximum lift, but improving the 
ramping and increasing duration and valve overlap.

Unfortunately, there's no free lunch unless you can graft on one of the new 
variable valve timing mechanisms. I'm referring to the V V T systems which 
change valve timing at around 3-4,000 rpm, not the emission driven intake 
retard systems used on the later Spider 4. The latest V V T systems can 
also vary lift.

For the V6 I am sceptical about putting S cams on an L engine for example. 
If all you do is gain 4 lb ft of torque at higher rpm, and another 10 or so 
hp at peak output rpm (around 5,500 rpm) and give up lower end torque, the 
L with it's taller final drive will probably accelerate less well. The 
extra hp might not even produce much difference at top speed either. 
Already capable of 240 km/hr what is the advantage in NA to a car that 
might do 250 km/hr (that would be a big gain for only 5% more power)?

Changing valve timing invariably detracts from some aspect  of engine 
performance, usually low end torque is sacrificed for higher end torque, 
which produces more power due to the rpm multiplier. Often this makes the 
car slower in normal street traffic.

The Honda 2000 is a case in point. Even with a very sophisticated V V T the 
240 hp is only developed at close to 8,000 rpm. Peak torque is only a few 
thousand rpm lower (at around 6-7,000 I think) so the engine is a bit of a 
dog around town. 2,000 cc is only capable of generating so much torque at 
any given rpm, even with the most sophisticated engine management and 
clever valve gear. As for adding Weber carbs, careful thought is required 
before fiddling with factory specs on cam timing. Only rarely should 
increasing valve lift be viewed as a suitable modification, given all the 
alternative choices.

Cheers everyone


Michael Smith
Calgary, Alberta
Canada
91 Alfa 164L, White, original owner

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